The present invention relates to an improved method of making an optical fiber perform having at least one region formed of a fluorine-doped glass.
Vapor deposition of doped silica is the most commonly employed technique for forming optical waveguide fibers. Such fibers are usually doped with GeO.sub.2 or P.sub.2 O.sub.5 to form a glass having a refractive index greater than that of silica or with B.sub.2 O.sub.3 or fluorine to form a glass having a refractive index less than that of silica. Because of its low absorption at long wavelengths, fluorine is preferred over B.sub.2 O.sub.3 for transmission at wavelengths beyond approximately 1.2 .mu.m. Fluorine has been employed as the sole dopant in single-mode fibers having a silica core and a fluorine-doped silica clad. Fluorine has also been added along with other dopants in the core of a single-mode fiber to change the zero dispersion wavelength, and it has been added to other dopants to obtain the desired combination of properties such as refractive index and viscosity. For example, fluorine and P.sub.2 O.sub.5 can be added to silica to form a diffusion barrier having the same refractive index as a silica substrate tube. However, the inclusion of fluorine as a dopant in vapor deposited glass has caused some disadvantages. U.S. Pat. No. 4,335,934 reports that fluorine tends to reduce the rate of deposition of doped silica glass on the inner surface of a substrate tube. It has been found that the addition of a fluorine-containing compound to the reactant stream emitted by a flame hydrolysis burner tends to decrease the rate of deposition of glass soot collected on the mandrel. Also, seeds were commonly formed in the resultant article when both fluorine and germania were co-deposited with silica.
A further disadvantage was experienced while attempting to form fluorine-doped silica by supplying C.sub.2 F.sub.6 to a flame hydrolysis burner. Even though the amount of C.sub.2 F.sub.6 was increased, the amount of fluorine in the resultant glass could not be increased to more than 0.6 wt. %. It is thought that a fluorine-doped silica particle is not immediately formed; rather, the fluorine must diffuse into the silica particle as it travels from the burner to the soot preform. Such diffusion must take place within a fraction of a second. The partial pressure of fluorine adjacent to the silica particle is very low since the fluorine supplied to the flame diffuses into the ambient atmosphere. Furthermore, some of fluorine adjacent the silica particle reacts with hydroxyl ions present in the flame to form HF; this fluorine is no longer available to dope the particle.